In measuring and testing systems, it is at times necessary to provide a selectable sequence of tones in as short an interval as possible. Such tones should have a predetermined calibrated amplitude which is relatively free of temperature and aging effects which commonly affect transmission characteristics of circuit elements such as coupling transformers. Known tone generating circuits have analog feedback control arrangements, but these usually require a significant settling interval each time that a particular tone is called for. The result is a corresponding lengthening of the time required to produce a sequence of at least one occurrence of each of a plurality of different tones of calibrated amplitude. Separate continuously running tone generation circuits could be employed for the respective tones required and wherein each such circuit includes its own stabilizing control. However, this requires substantial generating circuitry thereby increasing the size and weight of the test equipment including such tone generators. The circuit complexity can be reduced to some extent by switching outputs of the tone generators to provide the respective tones through a common output interface coupling device, such as a transformer. However, such a device is often one of the principle sources of the offending temperature and aging effects; and its contribution to the effects would not be compensated by the operation of individual generator stabilizing feedback circuits that do not include it.
Various circuits are known in the art which include some digital circuit portions in an amplitude control circuit. Two examples which utilize digital counting links are shown in the R. P. Chambers et al. U.S. Pat. No. 4,066,977 on an automatic gain control amplifier and the R. Treiber U.S. Pat. No. 4,161,633 on a line circuit with a regulated signal generation function. However, the systems in these patents must be initialized, i.e., recalibrated, each time that a discrete new signal is to be produced.
It is also known as shown, for example, in the H. G. Alles U.S. Pat. No. 4,201,105, to store digital characters representing successive sample magnitudes of a known waveform and read these digital samples out through a multiplier when it is desired to produce a wave of the same basic configuration with a selectable amplitude. However, the Alles disclosure does not deal with the amplitude calibration problem.